Since lithium secondary batteries have great electrochemical capacity, high operating potential and superior charge/discharge cycles, demand therefor in the fields of portable information terminals, portable electronic devices, small power storage devices for home use, motorcycles, electric cars, hybrid electric cars, and the like is increasing. Hence, improvements to the safety and performance of lithium secondary battery are required in response to the proliferation of such applications.
Conventional lithium secondary batteries using a liquid electrolyte are problematic because of poor stability owing to easy ignition upon exposure to moisture in the air. Such problems pertaining to stability come to the fore as electric cars are becoming popular.
In order to improve safety, thorough research is thus ongoing these days into all-solid-state secondary batteries using a solid electrolyte composed of a non-combustible inorganic material. All-solid-state secondary batteries, having stability, high energy density, high power output, long life, simple manufacturing processes, large/small sizes, and low prices, are receiving attention as a next-generation secondary battery.
An all-solid-state secondary battery is configured to include a cathode, a solid electrolyte layer and an anode, in which the solid electrolyte of the solid electrolyte layer has to possess high ionic conductivity and low electronic conductivity. Furthermore, a solid electrolyte is contained in the cathode and the anode as electrode layers, and the solid electrolyte used for the electrode layers is ideally formed of a conductive material mixture having both high ionic conductivity and high electronic conductivity.
A solid electrolyte that satisfies the requirements of the solid electrolyte layer of the all-solid-state secondary battery includes a sulfide, an oxide, or the like. In particular, a sulfide-based solid electrolyte is problematic in terms of production of a resistance component through the interfacial reaction with a cathode active material or an anode active material, high moisture absorption properties, and also generation of a hydrogen sulfide (H2S) gas that is poisonous.
Japanese Patent No. 4,779,988 discloses an all-solid-state lithium secondary battery having a layer structure comprising a cathode/a solid electrolyte layer/an anode, the solid electrolyte layer being formed of a sulfide.
Known examples of an oxide-based solid electrolyte may include LLTO (Li3xLa2/(3-x)TiO3), LLZO (Li7La3Zr2O12), and the like, among which LLZO, having relatively high grain boundary resistance but superior potential window properties compared to LLTO, is receiving attention as a prominent material.
LLZO, having high ionic conductivity, low reactivity with an electrode material, a wide potential window (0-6V), and the like, is disadvantageous because processing conditions are difficult to control owing to the volatilization of lithium (Li) during a sintering process and also because actual application thereof is difficult because of complicated manufacturing processes due to the difficulty in sintering thereof. Moreover, there is a significant difference in ionic conductivity depending on the crystal structure, and therefore the development of a technique for adjusting the compositions of starting materials, sintering properties, and the like to thereby control the crystal structure of LLZO is required.